The present invention concerns a novel process for the preparation of methylenebisphosphonic acids of the formula I ##STR3## wherein Q.sup.1 and Q.sup.2 are independently hydrogen or halogen, by hydrolyzing the corresponding methylenebisphosphonic acid tetraester of the formula ##STR4## wherein R has the meaning of a straight or branched alkyl group containing 1 to 4 carbon atoms, and Q.sup.1 and Q.sup.2 have the same meaning as above.
Prior known processes for the preparation of bisphosphonic acids by hydrolyzing the corresponding tetraesters have been based on the use of strong acids, such as halogen acids. Thus tetraesters, e.g. the isopropyl tetraester, have been boiled for several hours with concentrated hydrochloric acid (c.f. BE-patent 672205, example VI (B)). In the publication Houben-Weyl, Methoden der Organischen Chemie, XII, 1, 352-356, the hydrolysis of tetraesters with half-concentrated hydrochloric acid under elevated pressure and at 130.degree.-145.degree. C. is described. A disadvantage with these known processes is the impurity of the products obtained and the numerous purification steps needed to remove byproducts and excess acid. In addition, the processes are connected with substantial corrosion problems. From the EP-patent application 0200980 is known a process according to which methylenebisphosphonic acids have been prepared by hydrolysing tetraesters with plain water, by boiling at reflux temperature of the reaction mixture. In this case the above mentioned problems of impurities and corrosion are largely overcome, but another problem is the very long reaction time, which may be as long as 16 hours, which is disadvantageous, i.a. from the viewpoint of production economics.